The invention is directed to cameras of the type producing images on photographic film. While the teachings of the present invention are applicable to many types of cameras utilizing various film sizes, for purposes of an exemplary showing, the invention will be described in its application to a conventional, 35 mm, single reflex camera, since such cameras are the most widely used today.
As used herein and in the claims, the term "full-frame" is intended to refer to the film image area and shape for which any given camera is designed. In the conventional 35 mm camera, this full-frame image area constitutes a rectangle 24 mm by 36 mm, with the long dimension of the image area extending in the direction of advance of the film. Generally, 35 mm film is available in cassettes containing lengths of film to nominally provide 20, 36 and (more recently) 72 full-frame image areas or "exposures". Cameras of the type under consideration are usually characterized by the fact that the film advance mechanism is permanently interlocked with the shutter cocking mechanism (and any other necessary functions to be performed in readying the camera for a new exposure).
In recent times, the cost of film has escalated significantly, largely due to the increasing price of metallic silver, a critical constituent of the contemporary light-sensitive emulsion coating on the film's structural base. To the increasing cost of the film there must, of course, be added the corresponding increased cost of processing.
Furthermore, there has recently been an unprecedented proliferation of reasonably priced power winders and motor drives. Both types of devices enable the rapid advancement of film through the camera. The difference between power winders and motor drives is primarily a matter of convenience and versatility. Motor drives are generally more sophisticated, providing more modes of operation and higher "framing" rates (typically 5 to 6 frames per second as opposed to 1.5 to 2 frames per second for power winders). While these devices have become quite popular, they generally result in a large increase in the amount of film used, adding considerably to the cost of taking pictures. For example, when a motor drive having a firing rate of 5 or more frames per second is used, a 36 exposure film cassette can be exposed in less than 8 seconds. Heretofore, the only recourse has been to use a bulk film back fitted on the camera, holding 250 full-frame exposures on roughly 33 feet of film. Such equipment is quite expensive, is available for only a very few camera models, and adds considerably to the weight and bulk of the camera. More recently, as indicated above, a roll of 72 exposure 35 mm film in a conventional sized film cassette as been made available, by utilizing an extremely thin 3 mil film base.
The conversion means and method of the present invention enable a conventional 35 mm full-frame camera to produce half-frame images. As used herein and in the claims, the term "half-frame" refers to a film image area and shape constituting 1/2 of a full-frame image. The present invention enables a conventional camera to produce at least twice as many exposures as presently designated for a given length of film. Thus, on a conventional 36 exposure 35 mm film, from 72 to 76 exposures can be produced. Similarly, on a 72 exposure 35 mm film from 144 to 150 exposures can be obtained. As a result, the photographer is free to concentrate on his subject, composition, etc with less worry about running out of film. While not intended to be so limited, it will be immediately apparent that the conversion means and method of the present invention are particularly advantageous when applied to a camera having a power winder or motor drive. This is true without having to add to the weight or bulk of the camera. The camera, modified in accordance with the present invention, feels, appears and handles exactly as it does in its unmodified condition. The invention circumvents the interlocked film advance and shutter cocking mechanisms without necessitating alterations or modifications in these basic mechanisms.
For those interested in taking a lesser number of pictures, a conventional 20 exposure film will yield from 40 to 44 exposures. Many individuals prefer to purchase film in bulk, and load their own cassettes. Under these circumstances, the choice of number of photographs is virtually unlimited. For example, a "bulk" loaded length of film equivalent to 12 full-frame exposures, will yield from 24 to 28 half-frame exposures.
The invention not only results in the saving of film and processing chemicals, but also in the correlative reduction of pollution, resulting from improper disposal of processing chemicals, often by the increasing number of individuals doing their own processing.
The half-frame image differs from the full-frame image in more than just area. The long dimension of the half-frame image is oriented perpendicularly to the long dimension of a full-frame image. Significantly, the aspect ratio (ratio of the short dimension to the long dimension ) of the half-frame image differs considerably from the aspect ratio of the full-frame image. With conventional 35 mm film, a half-frame image is 18 mm by 24 mm. Thus the aspect ratio of a half-frame image is 3:4. This is much closer to the aspect ratios of standard printing paper formats (generally 4:5) and the 1:1 aspect ratio of standard square projection screens.
A full-frame image has an aspect ratio of 2:3. When a full-frame image is enlarged to fully cover a standard 8".times.10" sheet of printing paper 17% of the image is cropped. Similarly, when a full-frame image is projected so that its long dimension matches the width of a square projection screen, 1/3 of the screen remains dark and unused.
On the other hand, when a half-frame image is enlarged to fully cover a standard 8".times.10" sheet of printing paper, less than 6% of the image is cropped. Similarly, when a half-frame image is enlarged so that its long dimension fills a conventional square projection screen, only 25% of the screen is left dark. Thus it can be seen that the half-frame format is more efficient both in printing and projecting.
Heretofore, prior art workers having devised so-called "half-frame cameras". These are cameras utilizing standard 35 mm film and producing thereon a plurality of consecutive, side-by-side, half-frame images. The lenses for such cameras are tailored for the half-frame image, and the film advance mechanism is designed to advance the film 1/2 frame per actuation. When compared to then-contemporary full-frame cameras, these half-frame cameras were more compact and lighter in weight. They were characterized by all of the advantages thus far enumerated.
Nevertheless, through technological advance and the use of new materials, present day full-frame 35 mm cameras are comparable in compactness and weight to the earlier half-frame cameras. Half-frame cameras are not in wide-spread use today, although the formats intrinsic superiority is still being extolled.
The conversion means and method of the present invention has a number of advantages not found in the so-called half-frame cameras. First of all, since the conversion of a camera to expose half-frame images in accordance with the present invention requires no primary mechanical modification of the camera and is readily reversible; the camera may be used either in its full-frame mode or its half-frame mode.
In the prior art as applying to so-called "half-frame"mm cameras, both the camera and its lens are tailored to expose half-frame images. Thus, in such cameras, the half-frame image is indeed a full-frame image for that camera. This is true because, by the definition of "full-frame" above, the half-frame image area and shape are those for which the camera was originally designed. In predominent lens designs, optical performance tends to degrade as one proceeds radially outwardly from the lens axis toward the image periphery. In accordance with the teachings of the present invention, a conventional camera is converted in such a way that a half-frame image is produced, located centrally with respect to the normal full-frame image for which the camera was designed. Taking a conventional 35 mm single lens reflex camera as an example, 90% of the centrally positioned half-frame image lies within a locus defined by a 12 mm radius circle, centered on the lens axis. This 12 mm distance is equal to the distance from the lens axis to the near edge of the full-frame format. As a result, nearly optimum lens performance (resolution and contrast, lack of a variety of distortions, and minimal fall-off in illumination at the half-frame format corners) is achieved over the entire half-frame format.
The optimization of lens performance over the centrally positioned half-frame image area, together with the greater efficiency achieved in printing and projecting, easily offsets the effect of a physically smaller image area, while simultaneously providing at least more than double the number of photographic images for any given length of film.
For those cameras provided with focal plane shutter curtains travelling laterally (in the direction of film advance), an improvement in focal plane shutter speed accuracy and evenness of exposure across the film is achieved. This is true since the effects of acceleration and deceleration of each of the two shutter curtains, at their respective limits of travel, are minimized, since they will occur well to either side of the laterally reduced plane of the half-frame exposure.
As will be described hereinafter, the mask of the present invention may be so configured and made of such material, that it can be used to aid in stabilizing and flattening the film at the focal plane. Improved film flatness during exposure, increases image sharpness (i.e. resolution of detail on the film), as well as reducing the tendency on the film's part to oscillate in a mode analogous to a partially supported and often rapidly moving ribbon. Equally important, however, is the fact that there will be a smaller likelihood that perturbations of the film, during rapid motorized advance, could cause sufficient misalignment of the film's sprocket holes relative to the camera sprocket teeth, as to result in tearing of the film at the sprocket holes, or even jamming during such rapid advance by a winder or motor drive. It will be understood, that such stabilization and flattening of the film is even more advantageous when using the very thin base 72 exposure film.
Yet another advantage of the practice of the present invention, may be described as focal length parity. The concept of using differing focal lengths in order to achieve the same effective angle of view for different image formats, is based upon the fact that most camera lenses are designed in such a way that the image circle that camera lens produces at the focal plane, just covers the corners of the format for which the camera was designed. Thus, the major diagonal of the format is substantially equal to the diameter of the circular image at the plane of focus. This is primarily true for those cameras wherein the central optical axis remains fixed relative to the image format.
Since the present invention contemplates means and a method for the conversion of a conventional full-frame camera to a half-frame format, the focal lengths of lenses used with the camera (yielding relative designations as wide angle, normal and telephoto) will remain the same for either mode of use of the camera. Such parity would not be the case when comparing a conventional 35 mm camera to a prior art half-frame camera (designed for the half-frame format utilizing 35 mm film). In the latter camera, the image circle diameter at the plane of focus is substantially equivalent to the major diagonal of the half-frame format since, as has been stated above, the half-frame format is indeed the full-frame format for which such cameras were designed.